Ethylene is copolymerized with comonomers such as alkyl acrylate or alkyl methacrylate esters or vinyl esters, to create polymers with a different set of properties and attributes not obtainable from homopolymers themselves. Some attributes like adhesion and low temperature toughness, are significantly improved as the content of comonomer(s) is increased. However, increasing comonomer content invariably leads to lower peak melting temperatures, sometimes significantly lower peak melting temperatures, especially in copolymers containing more than about 5 mol % comonomer. As an example, commercially available ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA) grades show a significant decrease in peak melting temperature as comonomer content is increased, as shown in FIG. 1 (prior art). These lower peak melting temperatures indicate smaller crystallite sizes that result from shorter runs of uninterrupted, repeating ethylene units in the polymer's backbone. Thus, it is difficult to achieve high melting points with even moderate amounts of comonomer.
Peak melting points can also be significantly reduced by increasing melt index. Data for commercially available, nominally 28 weight % vinyl acetate (VA) copolymers shows a decrease from about 73° C. at 2.3 g/10 min melt index to 63° C. at 420 g/10 min melt index, as shown in FIG. 2 (prior art). These observed trends indicate that using conventional technology, high MI and high comonomer content copolymers will have very low melting points. In certain applications, however, such as hot melt adhesives and some film or molded articles, where heat resistance and strength are particularly important, it would be advantageous to have a copolymer having a combination of high melt index, relatively high comonomer content, and higher melting point.
The use of continuous, back-mixed autoclaves to produce homogeneous ethylene acrylate copolymers is disclosed in U.S. Pat. No. 3,350,372. Copolymer resins produced using autoclave technology are commercially available, and these resins are consistent with the expected trend. For example, as discussed more fully in the Examples herein, at 32.5 weight percent n-butyl acrylate (9.5 mol %), the peak melting temperature of a 330 g/10 min melt index ethylene n-butyl acrylate copolymer available as Enable™ EN-33330 from ExxonMobil Chemical, is poorly defined, with a value about 58.7° C. A lower viscosity grade, with an estimated melt index of about 900 g/10 min and available as Enable™ EN-33900 from ExxonMobil Chemical, has an even lower peak melting temperature of 58.1° C. (see Table 1).
Manufacturers have recognized that the lower melting points of commercially available copolymers are an undesirable limitation, and have attempted to develop technology to increase melting points of ethylene acrylate copolymers (EMA and EnBA) produced in continuous, high pressure autoclaves (see, e.g., U.S. Pat. Nos. 5,543,477 and 5,631,325). These technologies have led to copolymers with peak melting temperatures which are reportedly about 7 to 10° C. higher than other conventionally polymerized copolymers produced in autoclaves.
The first commercial, continuous process developed to produce ethylene alkyl acrylate and alkyl methacrylate copolymers was a tubular reactor process developed by Union Carbide (see U.S. Pat. No. 2,953,551). Tubular reactors are known to be capable of producing ethylene alkyl acrylate and alkyl methacrylate ester copolymers having higher melting temperatures than the same copolymers polymerized in a high pressure autoclave. Commercially available ethylene ethyl acrylate copolymers, produced and sold by Union Carbide since the early 1960's, have relatively high peak melting temperatures compared to autoclave polymerized ethylene methyl acrylate copolymers at the same mole percent comonomer. However, the melting points of copolymers obtainable from these technologies are still undesirably low for some applications.
For further background information, see, for example, WO 00/58093; WO 97/34939; U.S. Pat. No. 5,543,477; DE 3217973; DE 3404744; EP 245773 and EP 575873.